I added ammo storage using plastic sheet. The pads are milliput. I added ammo boxes, the radio and other stuff in locations specified in interior diagrams I've seen. Military Modelling had an article on the M3 by Steve Zaloga with many helpful pictures.
The ammo storage lockers are too big in my version. There should be some space against the back firewall. Nonetheless, the new interior should look better than the inaccurate kit contents.